1. Field of the Invention
This invention relates to structural cooling and particularly to a new and improved multiple-impingement cooled structure, such as for use as a turbine shroud assembly.
2. Description of the Prior Art
Structures, such as turbine shrouds and nozzle bands, which are subjected to high temperatures must be cooled in order to reduce possible damage caused by undesirable thermal expansion and to maintain satisfactory sealing characteristics. Several methods of cooling such structures are currently being successfully employed.
One method is film cooling. In film cooling, a thin film of cooling fluid, such as air, is directed to flow along and parallel to the surface which is to be cooled. Although film cooling provides excellent cooling, when used adjacent a gas stream, such as along the inner surface of a turbine shroud in the turbine section of an engine, the film cooling air mixes with the gases in the gas stream. The momentum of the film cooling air is lower than the momentum of gases with which it mixes and thus the resultant overall momentum of the mixed gas stream is lowered. Also, the mixing of the film cooling air with the gases in the gas stream imparts some turbulence to the gas stream. The net result of the mixing of the film cooling air with the gas stream is, in the case of the turbine section of an engine, that there is less work available to rotate the turbine rotor and thus turbine efficiency is decreased. Correspondingly, the greater the amount of film cooling air used, the greater will be the turbine efficiency decrease caused by mixing losses.
Another method of cooling structures is impingement cooling. In impingement cooling, air is directed to impinge substantially perpendicularly upon the surface of a structure to be cooled. When used on a turbine shroud, for example, cooling air is directed to impinge upon the back or outer surface of the shroud, that is, the surface not facing the gas flowpath. The source of the cooling air for both impingement and film cooling air in most gas turbine engines is high pressure air from the compressor. For effective impingement cooling of the entire turbine shroud in current impingement cooling arrangements, a relatively large amount of cooling air must be employed and thus the compressor must work harder to supply the cooling air. Thus, when a large amount of cooling air is required for impingement cooling, engine efficiency is reduced.
In view of the above-mentioned problems, it is therefore an object of the present invention to provide a structure having a unique configuration whereby it can be satisfactorily cooled with a reduced amount of film cooling air to thereby reduce mixing losses.
Another object of the present invention is to provide a structure configured whereby impingement cooling air is directed to impinge more than once upon an element of the structure to be cooled, thus requiring a reduced amount of cooling air and thereby increasing engine efficiency.